1. Field
The exemplary embodiments generally relate to material handling devices and, more particularly, to an edge gripping robot arm end effector.
2. Brief Description of Related Developments
Generally conventional edge contact gripper designs for gripping, for example, semiconductor wafers or substrates, include a pair of front substrate rest pads with hard stops located at a distal end (e.g. the tips) of the end effector. A pair of rear substrate edge rest pads is positioned to support the other end of the substrate at the proximate end of the end effector. An active gripping member or pusher is disposed at the proximate end of the end effector and movable by, for example, a vacuum operated actuator between a retracted substrate loading position and an extended position that clamps the substrate against the hard stops at the tips of the end effector. The pusher contacts the edge of the substrate through a substantially flat slide surface incorporated into the tip of the pusher. The vacuum operated actuator includes a suitable arrangement for detecting the position of the pusher, which position is used to determine whether a substrate is present and/or properly clamped on the end effector.
When a substrate is loaded on the end effector, i.e. lifted from a substrate holding station, in an initial phase of a pick operation, the center of the substrate is typically offset from the longitudinal axis of the end effector. As the pusher extends to clamp the substrate, the substrate is pushed and slides forward toward the tips of the end effector until its edge contacts a first hard stop on one of the tips of the end effector. From this point in the pick operation, in order to properly complete the clamping phase of the pick operation, the substrate slides or rolls along the hard stop until the substrate is completely seated against the other (e.g. second) hard stop on the other tip of the end effector. In this seating process the edge of the substrate slips on the hard stop or on the flat surface of the pusher.
In order for the substrate to roll along the first hard stop, the force exerted on the substrate by the pusher generally produces a moment in the counterclockwise (or clockwise) direction so that the substrate rolls along the first hard stop until it is completely seated against the second hard stop. Similarly if the substrate contacts the second hard stop before the first hard stop the force exerted on the substrate by the pusher causes the substrate to roll along the second hard stop until the substrate is completely seated against the first hard stop. The direction of force exerted on the substrate by the pusher may be determined by the coefficient of friction between the edge of the substrate and the flat side surface of the pusher. This is generally because the force can be decomposed into a component normal to the edge of the substrate and the flat side of the pusher at the point of contact, and a frictional component tangential to the edge of the substrate and the flat side of the pusher. The ratio of the magnitude of the tangential frictional component and the normal component corresponds to the coefficient of friction.
In one example, where the first hard stop is in contact with the edge of the substrate, if the coefficient of friction increases, e.g. as a result of a contamination buildup on the flat slide surface of the pusher, the direction of the force exerted by the pusher on the substrate changes towards the point of contact between the edge of the substrate and the first hard stop. When the coefficient of friction reaches a level at which the direction of the force exerted on the substrate by the pusher passes through or to the left of the point of contact between the substrate edge and the first hard stop, the gripper locks up (e.g. stops moving such that the substrate is not properly seated or positioned on the end effector) so that the substrate will not move towards the other hard stop.
It would be advantageous to have an edge contact gripper that substantially eliminates gripper lock up problems resulting from increased friction between the pusher and the substrate edge due to, for example, contamination buildup.